Association of extreme heat events with sleep and cardiovascular health: A scoping review

BACKGROUND: Extreme heat events (EHEs), driven by anthropogenic climate change, exacerbate the risk of cardiovascular disease (CVD), although the underlying mechanisms are unclear. Disturbances in sleep health, caused by excessive heat, may be one way EHEs increase the risk of incident or recurrent CVD. Our objective was to systematically review the empirical peer-reviewed literature on the relationship between EHEs, sleep health, and cardiovascular measures and outcomes, and narratively describe methodologies, evidence, and gaps in this area. METHODS: A comprehensive literature search was performed in the following databases from inception – June 2023: Ovid MEDLINE, Ovid EMBASE, CINAHL, Web of Science and The Cochrane Library. Studies retrieved were then screened for eligibility against predefined inclusion/exclusion criteria. RESULTS: Of the 2035 records screened, three studies met the inclusion criteria. Cardiovascular (CV) measures described included blood pressure (BP), heart rate (HR), and HR variability (no CVD outcomes were described) and objective and subjective measurements of sleep health outcomes included sleep duration, calmness, ease of falling asleep, ease of awakening, freshness after awakening, and sleep satisfaction. Two studies were controlled trials, and one was a cohort study. During EHEs, individuals slept for shorter periods of time and less efficiently, with greater degrees of HR variability in two of the three studies lasting at most 1–2 days; BP (both systolic and diastolic) significantly decreased during EHEs in two of the studies. No formal assessment of a mediating relationship between EHE exposure, sleep outcomes, and the CV measures was undertaken. CONCLUSIONS: There is a paucity of data that examines the link between CVD, sleep, and extreme heat as a possible mechanism of elevated CVD risk during EHEs, despite a strong physiological rationale. Further research is needed to empirically test this relationship rigorously as EHEs become more frequent and their deleterious impacts of health increase.


Background
Extreme heat events (EHEs) are periods of unusually high temperatures, which are increasing in frequency, intensity, and duration as a result of anthropogenic climate change. 1 While no single de nition of EHEs exists, 2 since 1950, the number of heatwave days (de ned as at least three consecutive days above the 90th percentile of daily maximum temperature) are estimated to have increased by 2.26 per decade globally, while the cumulative heat (i.e., the extra heat produced by a heatwave over a given season) increased by 2.84 ºC per decade. 3Heat exposure is associated with adverse cardiovascular (CV) events, with every 1ºC rise in ambient temperature signi cantly raising the risk of cardiovascular disease (CVD)-related morbidity and mortality. 4ough increased CVD risk due to EHEs have been thoroughly documented in the literature, the mechanisms leading to associated CVD morbidity outcomes remain unclear. 5isting research describe a surge in cardiac output and hyperventilation during EHEs, yet the speci c pathways leading to CVD exacerbation during EHEs are not fully delineated. 6,7,8One potential mechanism linking EHEs and CVD is sleep disruption.Rising temperatures have been associated with shorter sleep duration and poorer sleep quality, as has the aftermath of weather phenomena impacted by climate change like hurricanes, oods, and wild res. 9,10,11,12ltiple dimensions of sleep health, including insu cient sleep duration, irregular sleep schedules, and poor sleep quality, can increase cardiometabolic risk predisposing to CVD. 13 For instance, poor sleep has been linked to higher risk for hypertension, 14,15 obesity, 16 and type 2 diabetes 17 via theorized mechanisms including in ammation 18,19 , glycemic dysregulation, 20 increased sympathetic tone via increases in nocturnal catecholamines. 21Thus, this connection suggests a possible mechanism explaining the adverse impact of EHEs on CVD, with sleep as a mediating factor, as illustrated in Fig. 1.Therefore, in this scoping review, we systematically examined the peer-reviewed literature that examined the relationship of EHEs with sleep health, and CV measures and CVD outcomes.

Methods
This study was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses-Scoping reviews (PRISMA-ScR). 22In adherence to this statement, a protocol was registered with PROSPERO International prospective registry (PROSPERO; CRD42023432124).

Search Strategy
A medical librarian (MRD) performed comprehensive searches to identify studies that examined the effect of EHEs (heat waves) on sleep health and CVD.
Searches were completed on June 14, 2023 in the following databases: Ovid MEDLINE (ALL − 1946 to Present); Ovid EMBASE (1974 to present); CINAHL (EBSCO); Web of Science (Core Collection -Clarivate); and The Cochrane Library (Wiley).The search strategy included all appropriate controlled vocabulary and keywords for the concepts of "heat," "sleep," and "cardiovascular."The full search strategies for all databases are available in an additional le.In order to limit publication bias in our initial search strategy, there were no language, publication date, or article type restrictions on the search strategy.

Study Selection
Retrieved studies were screened for inclusion using Covidence systematic review software.Titles and abstracts were reviewed against prede ned inclusion/exclusion criteria by two independent reviewers.Discrepancies were resolved by consensus (NA, SW, RA, MRD, MC, AKG).For nal inclusion, full text was then retrieved and also screened by two independent reviewers.Our inclusion criteria were articles that included the following: (1) EHEs, as de ned by manuscript-speci c de nitions; (2) Reported sleep measures: sleep health disruptions (e.g., sleep duration ≤ 7 hours, irregular sleep, di culty falling asleep, symptoms of sleep disorders, and/or daytime sleepiness); (3) CV measures (e.g., blood pressure, heart rate) and CVD events/diagnoses (e.g., diagnosis of hypertension, coronary artery disease, and peripheral arterial disease or acute CVD events such as myocardial infarction, stroke, and heart failure exacerbations); and (4) Adult participants ( > = 18 years).Excluded studies were: (1) Non-English; (2) Review articles, commentaries, viewpoints, editorials, or case reports; (3) Insu cient CV measures or outcomes; (4) Insu cient measure of sleep de ned; or (5) Lack of EHE or equivalent, or lacks de nition of EHE.For articles selected for inclusion in this study, reference lists and citing articles were pulled from Scopus (Elsevier) and also screened.The full PRISMA ow diagram outlining the study selection process is presented as Fig. 2.

Summary of Articles
Three studies, summarized in Table 1, met criteria for inclusion in the analysis.Two studies were controlled trials, 23,24 while one was an observational cohort study. 25Huang et al. recruited participants (n = 41) into one of three intervention groups or a control group to assess the effect of these interventions (subsidies for air conditioning, education about health and environmental heat, and use of sprinklers to cool home exteriors) on sleep quality and CV measurements during a heatwave. 23Yan et al. performed a controlled, cross-over trial, placing participants (n = 16) in one of four room permutations, with rooms either 27ºC or 30ºC and either employing mechanical ventilation systems to circulate ltered outdoor air or not. 24Kim et al. observed the effects of a heat wave on elderly residents of rural communities in South Korea (n = 104). 25

Geographies covered and study settings
The three studies analyzed here report data from China, 23,24 and South Korea 25 .One of these studies was at the regional level, 25 while two were at the city level. 23,24 Extreme Heat Event De nitions De nitions of EHEs varied greatly.In the Xinyi study, EHEs were de ned as heatwaves using the 90th percentile daily maximum temperature.23 In the South Korea study, the criteria for heatwave were more than two consecutive days with a maximum temperature more than 33º C. 25 Finally, the Shanghai controlled trial used indoor temperatures of 27º C and 30º C to model extreme heat exposure.24

Sleep Outcomes Evaluated
Sleep outcomes used in the studies were both self-reported and objective.Subjective outcomes reported were self-assessed sleep duration, 25 selfassessments of calmness, ease of falling asleep, ease of awakening, freshness after awakening, and sleep satisfaction. 24Objective outcomes were total sleep duration (split into deep sleep duration and light sleep duration) as measured by a smart band, 23 and total sleep time, sleep e cacy (ratio of time asleep to time in bed), sleep onset latency (the time between turning off lights and falling asleep), time awake and duration of sleep stages (NREM sleep of stage N1, N2, and N3, and REM sleep), measured using electroencephalogram (EEG), bilateral electrooculogram (EOG), and chin electromyogram (EMG). 24

Cardiovascular Measures and Outcomes Evaluated
No CVD outcomes were evaluated in these studies.However, CV measurements were assessed.All CV measurements were assessed with objective data.These data included systolic (SBP) and diastolic blood pressure (DBP) 23,24,25 measured with a sphygmomanometer and HR 23 and HR variability 24 measured with an electronic wrist monitor or ECG, respectively.

Qualitative Synthesis:
None of the studies included in this review directly evaluated measures of sleep health as a mediator or confounder in the relationship between extreme heat and CVD, or CV measures.However, indirectly, Yan et al. showed a signi cant mean difference (MD) in HR variability (MD = 0.7 beats per minute [bpm]; p = 0.02) between 27ºC and 30ºC room at the same time as a signi cant decrease in total sleep time (MD = 39.1 minutes, p = 0.01), sleep e ciency (MD = 8%, p = 0.01), and REM sleep time (MD = 3.3 minutes, p = 0.05) and increase in time awake (MD = 38.1 minutes, p = 0.01) in rooms lacking mechanical ventilation (i.e., fans to bring in ltered outdoor air); as well as a decrease in sleep e ciency of 0.2% per increased bpm (p = 0.04) and an increase in time awake of 2.39 minutes per increased bpm (p = 0.04). 24m et al. present ndings suggestive of a relationship between extreme heat and both sleep and CV measures, however these neither reached statistical signi cance nor implied that sleep played a mediating role between extreme heat and CV measures.DBP decreased signi cantly (p < 0.001) in subjects with hypertension, with a 1ºC increase in indoor temperature decreasing DBP by 0.44 mmHg (95% CI: 0.04-0.84mmHg).The association between indoor temperature and SBP was positive but not signi cant.The number of hours of sleep decreased with indoor temperature by 0.036 hours (95% CI: -0.138, 0.067 hours), however this result did not reach statistical signi cance. 25milarly, Huang et al. show an association between extreme heat and both sleep and CV measures, though no causal mechanisms can be inferred.In the control group, DBP and SBP elevated from baseline during the heatwave, with SBP increasing signi cantly on days 1 and 2 by 5.33 mmHg (95% CI: 3.38-7.30;P = 0.01) and by 4.92 mmHg (95%CI: 2.74-7.09;P = 0.02), respectively.HR elevated from baseline and on day 1 and lowered to near-baseline by day 5.

Discussion
This scoping review identi ed three articles examining the relationship between extreme heat, sleep, and CV health.These papers did not elucidate a mechanism linking extreme heat to worsening CV measures or CVD.Their ndings are, however, largely in concordance with previous literature indicating that extreme heat adversely affects CV health, notably elevating BP, 4,5 and that it lowers measures of sleep quality. 11,12portantly, our review highlighted several current research challenges linking climate-ampli ed EHEs, sleep health, and CVD health.First, the studies that met our inclusion criteria suggest potential associations between heat, sleep, and CVD, yet fall short of delving into the underlying physiological mechanisms that might explain these associations.Second, the duration of the included studies was limited, the longest being 19 days. 25The limited follow-up period after the de ned EHEs may constitute an underlying bias of these studies, given research suggests adverse CVD outcomes can last up to 2 weeks after the antecedent EHE event. 26,27Further research involving a longer follow-up post EHE could help elucidate the connection between sleep and CV health, examining the time course of potential returns to baseline.Indeed, such studies could provide insight into opportunities to examine acclimatization strategies.Third, while duration and quality of sleep were examined, future research could evaluate multiple dimensions of sleep health and their relationship to EHEs and CVD, including timing and alertness.Last, studies lacked the inclusion of participants from at-risk populations, such as those with underlying CVD or whose surrounding environments render them particularly exposed to extreme heat.Research including these populations could offer perspective on ways to aid those most vulnerable to harm in a warming climate.Furthermore, the existing studies were limited in its geographic reach and lacked standardization of key terminology such as EHE de nition.Given that all of the studies reviewed were conducted in East Asia, our analysis underscores the need for a broader geographic representation in research on this subject.Further analyses may show important regional variation: given that the impacts and manifestations of climate change are so variable, associations between EHEs, sleep, and CVD should be investigated in different settings to inform interventions and policy; and, research conducted in speci c countries may nd an association between EHEs, sleep, and CVD more likely, particularly where CVD outcomes are more prevalent. 28The need for a uniform characterization of what constitutes an EHE or heatwave is also important in order to promote coherence and comparability between studies. 2 This would facilitate a more nuanced understanding of the broader implications of EHEs on health on a global scale, and allow comparisons across regions and countries.
Interestingly, the role of air conditioning (AC) as a potential confounder in the relationship between EHEs, sleep health, and CVD is likely an area for further study.In the studies conducted by Huang 23 and Yan, 24 a signi cant emphasis is placed on the role of interior room temperatures, positing them as more critical determinants of health outcomes than external temperatures.Their ndings hint at the potential bene ts of regulating indoor temperatures to enhance both sleep quality and CV health, thereby mitigating some of the adverse effects of EHEs.Notably, Huang's research delineates that daytime exposure to heat does not su ce to induce elevated SBP, suggesting that nocturnal temperature levels particularly may play a pivotal role -an area of future research. 29e role of AC further highlights the important relationship between EHEs and health inequities.Sleep health disparities have been shown to explain up to 50% of cardiometabolic disparities in some studies. 30EHEs may exacerbate health inequities, especially given that populations in low resource settings may not have infrastructure to adapt to extreme heat.One example of this is in New York City, where there exists a 10 percentage point gap in the rates of AC scarcity between White and Black residents. 31Future research on the relationship between EHEs, cardiovascular health, and sleep health could examine the role of health inequity and accordingly suggest policy interventions to reduce inequities in both sleep and CVD outcomes, particularly in relation to EHEs.
Although we conducted a comprehensive review, the nal number of results in very small.We argue that this underscores the early stage of research in this area, highlighting the need for more extensive and in-depth studies to build a robust body of evidence that can inform policy and public health interventions effectively to improve sleep, and cardiovascular health in the age of climate change.
We describe two limitations to our work.First, we did not include the non-English literature and therefore may have missed important peer-reviewed manuscripts.This is particularly noteworthy because of the three studies that met inclusion criteria, all were from countries outside the English-speaking regions.Second, we did not consider grey literature.However, given the nuanced nature of our research question and the conceptual framework we examined (Fig. 1), the peer-reviewed literature is the most likely source of empirical studies on this topic.

Conclusion
In conclusion, this scoping review examined the peer-reviewed literature that described the relationship between EHEs, sleep health, and CV health, an area of research that is still in its infancy.The existing literature, though limited, underscores the pivotal role of interior temperatures in in uencing health outcomes, hinting at the potential of targeted interventions to mitigate adverse effects.However, the current body of research is characterized by geographical limitations, and a lack of uniform de nitions and conceptual framework to understand the connections between sleep, heat, and CV measurements, all of which necessitate a more globally representative and cohesive approach in future studies.Moreover, the exploration of the underlying physiological mechanisms and the development of effective interventions remain largely untapped areas.Understanding these dynamics could suggest public health policies and strategies that mitigate the increasingly frequent effects of climate change in general, but particularly on CV health.

Figure 1 :
Figure 1: Hypothesized model of relationship between EHEs, sleep quality, and CV health.
Difference MRD: Medical Librarian PRISMA-ScR: Preferred Reporting Items for Systematic Reviews and Meta-Analyses-Scoping reviews SBP: Systolic Blood Pressure